1. Field of the Invention
The present invention relates in general to a pressing machine, and more particularly to a method and an apparatus for adjusting the operating conditions of a press on the basis of information of the machine and the dies used.
2. Discussion of the Prior Art
There has been widely used a press wherein a pressing operation such as a drawing operation is effected such that upper and lower dies removably installed on the machine are moved towards and away from each other. On some pressing machines of this type, the operating conditions may be changed or adjusted depending upon the specific set of dies used. In a single-action press constructed as shown in FIGS. 1 and 2, for example, a drawing operation on a workpiece or blank in the form of a metal strip or sheet is performed by an upper die 18 and a lower punch 12 while the blank is held by means of a pressure ring 30 in cooperation with the upper die 18. In this type of press, pressure control valves 46 and 84, a servomotor 60 and other components are provided to adjust the operating conditions such as the pneumatic pressures Pa and Pb of pneumatic cushioning cylinder 42 and counterbalancing cylinder 80, and distance h associated with a die-height adjusting mechanism 52. The pressure Pa of the cushioning cylinder 42 influences the holding force to be applied to the pressure ring 30, and the distance h influences the pressing force to be applied to the blank.
In a double-action press as shown in FIGS. 12-14, the holding force is applied to the blank through a pressure ring 156 attached to an outer slide 160, and the pressing force is applied to the blank through a punch 162 fixed to an inner slide 164 and a lower die 152 disposed on a bolster 154. In this double-action press, too, the operating conditions such as a pneumatic pressure Pe of a cylinder 184 which influences the holding force must be suitably adjusted so as to avoid the cracking and wrinkling of a product formed from the blank. The operating conditions to be adjusted also include a distance ha associated with a die-height adjusting mechanism 172.
The holding force and the pressing force which assure an adequate pressing operation without cracking and/or wrinkling of the products obtained differ depending upon the specific machine and the specific die set used on the machine. More specifically, the weights of the upper die 18, 162 and the pressure ring 30, 156 which cooperate with the lower die 12, 152 to constitute a die set vary from one die set to another. Therefore, it is necessary to suitably adjust the pressure Pa, Pe and the distance h, ha, for example, depending upon the specific die set. Further, the pressure-receiving areas of the cylinders 42, 184, and the sliding resistance and rigidity values of the machine components differ from one machine to another. Therefore, the operating conditions such as the optimum pressure Pa, Pe and distance h, ha for assuring an adequate pressing operation are generally determined or established by a trial-and-error procedure, namely, by performing test operations on the press to be used for production.
If the optimum pneumatic pressure Pa, Pe and distance h, ha for example are determined beforehand for different die sets during test operations on trial or test presses used for testing the individual die sets, the above-indicated trial-and-error procedure on the production press can be eliminated. As indicated above, however, the individual production presses have different operating characteristics, such as different pressure receiving areas of the cushioning cylinder 42, and different sliding resistance and rigidity values of the various components. Thus, the adjustment of the operating conditions according to the known or predetermined optimum values is not practically satisfactory for the individual pressing machines in general.
Further, the adjustment of the operating conditions according to the predetermined optimum values is likely to cause cracking and/or wrinkling of the product obtained by a pressing operation. This drawback is considered to arise from factors which relate to the properties of the blank such as elongation and surface roughness, and an amount of a lubricating oil deposited on the surface of the blank. Namely, even if the pressing operation is performed with the same blank holding force, the sliding resistance between the blank and the dies may fluctuate due to different properties of the individual blanks, which lead to different tensile forces acting on the different blanks during a drawing operation, for example. Although the material and thickness of the blank are taken into account when the optimum holding force is determined for each die set, the chemical composition and the thickness of the blank in the form of a metal strip prepared by rolling generally vary within certain ranges of tolerances. Further, a variation in the amount of deposition of the lubricating oil on the surface of the blanks is inevitable. Thus, there exist various fluctuating factors relating to the manufacturing process of the blanks, which cause different physical properties of the blanks.
The above analysis may suggest a necessity of determining the optimum pneumatic pressure Pa of the cushioning pneumatic cylinder 42, for example, by a trial-and-error procedure. In this respect, the relationship between the optimum pressure Pa and the optimum holding force on one machine is usually different from that on another machine. This means, the same amount of adjustment of the pneumatic pressure Pa is not necessarily adequate or sufficient to remove the same degree of cracking or wrinkling of the product. Accordingly, the adjustment is troublesome and time-consuming even for an experienced or skilled user or operator of the pressing machine. This is particularly so, on the double-action press in which the two or more cylinders 184 are used to adjust the blank holding force.
As described above, the known pressing machines require cumbersome and time-consuming trial-and-error procedure and a high level of skill for adjusting the operating conditions such as the pneumatic pressure Pa depending upon the specific die set, and are not capable of producing desired articles of manufacture with highly consistent quality.
A further study of the pressing operations on the pressing machines as described above suggests a problem that the blanks or products formed from the blanks are likely to easily crack or suffer from similar defects after a relatively long continuous pressing job in which a-given pressing cycle is repeated on a large number of blanks which are successively loaded on the machine. This problem is considered to occur due to a rise in the temperature of the die set by heat generated due to sliding resistance of the blank which is moved between and in contact with the lower and upper dies in the process of a pressing operation such as a drawing operation. The temperature of the die set rises as the number of the pressing cycles performed increases. As the temperature of the die set rises, the property of the oil lubricating the die set, characteristics of the die set and the friction characteristics of the blanks tend to vary, while the volatility of the lubricating oil increases. Consequently, the sliding resistance of the blanks with respect to the die set increases, causing an increase in the tensile force acting on the blanks during the drawing operation, and leading to easy occurrence of cracking or other defects of the formed products. The increase in the sliding resistance also accelerates the wearing of the die set, and shortens the life of the die set.
An analysis of the relationship between a sliding resistance of a blank or workpiece and the amount of heat generated by the sliding resistance indicates that an amount of heat Qo generated by a grinding operation is expressed by the following equation (a): EQU Qo=Ft.multidot.(Va.+-.Vb).multidot..tau. (a)
where,
Ft: tangential grinding resistance PA1 .tau.: grinding time PA1 Va: peripheral speed of a grinding wheel PA1 Vb: speed of movement of a workpiece to be ground PA1 W: amount of tangential movement of the blank PA1 w: width of the blank EQU W=a(.mu.+r).multidot..intg.f(t)dt (c)
In view of the above, the tangential sliding resistance in the case of a drawing operation performed on a blank on a press can be expressed as (.mu.+r).multidot.F(t), where .mu. represents a sliding resistance determined by the surface roughness of the die set and the blank and the lubricating condition, and r represents a resistance to a bend-back action of the blank due to a bead in the blank holding portion (pressure ring) of the die set, while f(t) represents a surface pressure of the blank holding portion. Hence, an amount of heat Qs generated in the blank holding portion is expressed by the following equation (b): EQU Qs=(.mu.+r).multidot.f(t).multidot.W.multidot.w (b)
where,
Therefore, the amount of heat Qs generated by the sliding resistance of the blank with respect to the die set is expressed by the following equation (d): EQU Qs=(.mu.+r).sup.2 .multidot.f(t).multidot..intg.f(t)dt.multidot.w (d)
On the other hand, the tensile force Te acting on the blank during the drawing operation is expressed by the following equation (e): EQU Te(.mu.+r).multidot.f(t) (e)
The above equation (e) indicates that the tensile force Te increases with an increase in the sliding resistance .mu. due to the temperature rise of the die set and the change in the lubricating condition of the blank, even if the surface pressure f(t) is held constant. Thus, the product formed from the blank tends to crack as the temperature of the die rises. It will be apparent from the above equation (d) that the amount of heat Qs also increases with an increase in the sliding resistance .mu.. Accordingly, there exists a vicious circle in which the temperature of the die set further rises, which in turn causes a further increase in the sliding resistance .mu..